1. Field of the Invention
The present invention relates to a thermoelectric material and a thermoelectric conversion module using the same.
2. Description of the Related Art
In recent years, with rising consciousness about the global environmental issues and the like, there has been growing interest in thermoelectric cooling devices using the Peltier effect, which are CFC-free refrigerators. Similarly, from the viewpoint of reduction in exhaust amount of carbon dioxide, and effective use of energy, there has been growing interest in thermoelectric power generating devices using the Seebeck effect, as a power generating system using unused waste heat energy. These thermoelectric conversion devices include a thermoelectric conversion module in which p-type and n-type thermoelectric materials are alternately connected in series.
As the thermoelectric material which is applied to the thermoelectric conversion module, a Bi—Te type single crystal and poly crystal are frequently used in the devices used at around a room temperature. When a thermoelectric conversion module is manufactured, both p-type and n-type materials are constituted of Bi—Te type materials. The n-type material is generally doped with Se. For a thermoelectric material used at a temperature higher than a room temperature, a Pb—Te type material is used because of high efficiency.
The Bi—Te type and Pb—Te type thermoelectric materials include Se (selenium), Pb (lead) and Te (tellurium) which are toxic and harmful to humans. These are also unfavorable substances from the viewpoint of the global environmental issues. Therefore, thermoelectric materials which are substituted for Bi—Te type and Pb—Te type materials are demanded, and the study on harmless thermoelectric materials is being carried out. As harmless thermoelectric materials, an Fe—V—Al type material having an L21 structure attracts attention (see JP-A 2004-119648 (KOKAI) and JP-A 2004-119948 (KOKAI)).
An Fe2VAl alloy which is the result of substituting V for ⅓ of Fe in Fe3Al has an L21 structure (a so-called Heusler structure), and shows the behavior of electric conduction of a semiconductor. The Fe2VAl alloy attracts attention as the material showing a high Seebeck coefficient, which is equivalent to that of an Bi—Te material, at a room temperature. Further, by substituting Si for a part of Al in the Fe2VAl alloy, the power factor (=α2/ρ, where α is a Seebeck coefficient of the thermoelectric material, and ρ is an electric resistivity of the thermoelectric material) of the thermoelectric material is enhanced.
However, the thermal conductivity of the Fe—V—Al type material is higher than that of the Bi—Te type material by about one order of magnitude, and this becomes the barrier to commercialization. The practical characteristics of the thermoelectric material are expressed by the figure of merit Z (=α2/(ρ·κ), where κ is a thermal conductivity of the thermoelectric material). Therefore, even if the power factor based on the Seebeck coefficient α and the electric resistivity ρ is high, when the value of the thermal conductivity κ of the thermoelectric material is high, the value of the figure of merit Z indicating practicality of the thermoelectric material cannot be increased sufficiently.